The present invention relates generally to a method of preparing thermally conductive mechanically compliant compounds for improving heat transfer from a heat generating semiconductor device to a heat dissipator such as a heat sink or heat spreader. More specifically, the present invention relates to a technique for preparing highly thermally conductive polymer compounds such as a polymer liquid loaded or filled with percolating particulate clusters coated with a liquid metal. Such compounds are highly effective through liquid metal enhanced percolation. More particularly, the present invention involves a process for uniformly coating particulate solids with a liquid metal, and thereafter blending the coated particulate with a liquid or fluid polymer for forming the compliant pad with thermal vias therein.
In the past, liquid metals have been proposed for incorporation in thermally conductive pastes for heat generating semiconductor devices. In most cases, the application of liquid metals for this purpose was not widely used, primarily because of problems created with the tendency of the liquid metal to form alloys and/or amalgams, thereby altering and modifying the physical properties of the liquid metal containing mounting pad. In addition, the highly thermally conductive pastes of the prior art are always electrically conductive which may not be desirable in certain applications and situations. In certain other situations, liquid metals and/or alloys of liquid metal were blended with a polymer, with the polymer thereafter being cured in order to provide a composite thermally conductive mounting pad. While useful, these devices did not find widespread application due primarily to the instability of the liquid metal component in the finished product. This instability is due to the extremely high surface tension as well as other chemical and physical properties of the liquid metal component. By way of example, the dispersed liquid metal droplets had a tendency to coalesce, a process of Ostwald ripening, and cause macroscopic separation of the metal from the polymer matrix.
The present invention utilizes the combination of a liquid metal coated particulate with a polymer carrier to prepare a thermal bridge having highly desirable thermal and electrical properties, and adapted to be configured to be interposed between a semiconductor device and a heat dissipator. The method of preparation described in the invention also renders the compounds highly stable in terms of macroscopic phase separation. In addition, the method of preparation renders possible the formation of large percolating clusters of liquid metal coated particles which enhances heat transfer. The combination also possesses desirable mechanical properties which facilitate its use in production operations.
In accordance with the present invention, a particulate such as boron nitride, alumina or aluminum nitride is initially dried, and thereafter placed in contact with a liquid metal, typically a metal that is liquid at room temperature or melting at a relatively low temperature, typically below 120xc2x0 C. and preferably below 60xc2x0 C. Preferably, the liquid metal comprises an alloy of gallium and/or indium, such as a gallium-indium-tin-zinc alloy, a bismuth-indium alloy or a tin-indium-bismuth alloy. In order to appropriately wet the surfaces of the particulate, a mixture of dried particulate and liquid metal is subjected to a mixing operation until the particulate is uniformly coated with the liquid metal. While not absolutely necessary, it is desirable that the boron nitride particulate be dry before blending with the liquid metal alloy. At this stage of mixing one obtains a thixotropic paste of liquid metal and the powder. One can also visualize the paste as a large percolating cluster.
Following the coating operation, the coated particulate is mixed with a liquid polymeric carrier material such as, for example, liquid silicone oil of a desired or selected viscosity. It is preferred that the liquid metal particulate be incorporated in the silicone mixture at or near the packing limit. For liquid metal coated boron nitride, the packing fraction is typically between about 60% and 65% by volume coated particles, balance liquid silicone. At these volume fractions, one obtains mechanically compliant compounds that have excellent thermal conductivity due to high packing density. This improves heat transfer due to the creation of a compliant interface between the opposed spaced-apart surfaces of the semiconductor device and the heat sink.
In preparing the mechanically compliant highly thermally conductive bridges in accordance with the present invention, the thermally conductive particulate is initially selected, with boron nitride being the preferred particulate. Materials such as aluminum oxide (alumina), and aluminum nitride have also been found to be useful when properly dried prior to contact with the liquid metal. For the application of the present invention, the particle size should be such that the average cross-sectional thickness is less than about 5 microns. A liquid metal, preferably a low melting alloy, is added to the particulate and mechanically mixed until the particulate surface is substantially uniformly wet by the liquid metal and a uniform paste is formed. Thereafter, a liquid polymer, preferably a liquid or fluid silicone polymer is added to the liquid metal paste to form a blend, with this blend being subjected to a mechanical mixing operation which typically includes a vigorous or high-speed mixing step, with vigorous mixing being continued until a visually smooth paste is formed.
When incorporated into liquid silicone, it has been found that the addition of the liquid metal coated particulate effectively reduces viscosity. The mechanism involved in this alteration of viscosity is believed to be due to the reduction of viscous drag at the xe2x80x9ceffective particlexe2x80x9d-silicone oil interface. The liquid metal coating increases the sphericity of the configuration of the particulate, and also contributes to an effective xe2x80x9csoftnessxe2x80x9d of the otherwise hard particles. These two factors function in a mutually cooperative fashion so as to reduce both viscosity and modulus of the resulting composite.
It has been further found that the liquid metal coated particulate, in addition to effectively transferring heat and/or thermal energy, also anchors the liquid metal into a three phase composite to prevent gross migration. The three phases are particle-liquid metal-polymer. By increasing the viscosity of the metal phase, the tendency of metal droplets to migrate and coalesce into large drops that could macroscopically separate and leak from the composite is severely retarded. Furthermore, it has been found that the liquid coated particulate provides a Bingham-plastic like character in the resultant composite, this allowing the paste to remain static in the absence of external stress, and yet conform and/or flow easily when subjected to stress.
Because of the tendency to undergo liquid-to-liquid macroscopic separation, liquid metals do not blend well with polymer liquids, including silicones. In accordance with the present invention, however, when particulate, in particular boron nitride, is initially coated with a gallium alloy, the microscopic separation phenomena is reduced, with the liquid metal being supported or retained in coated particulate form, due to the increased thixotropy of the metal phase. In addition, the coated particulate, when added to silicone, functions effectively to form thermal vias within the composite. In certain cases, the thermal conductivity of the particulate such as boron nitride, may even exceed that of the liquid metal, for example, a eutectic alloy of gallium, tin and indium.
It is a further feature of the invention that in addition to its thermal properties, the composite possesses desirable electrical properties as well. Formulations having the optimal thermal properties have been found to possess electrical volume resistivity in the range of 108 to 1012 xcexa9-cm.
Briefly, the technique of the present invention involves the steps of initially selecting a particulate material for the application. Boron nitride particles are particularly desirable, with those particles having a BET surface area of 0.3 m2-gxe2x88x921 have been found quite useful. Boron nitride is typically configured in the form of anisotropic platelet-like particles, with plate diameter ranging from about 5-50 xcexcm and the plate thickness being from about 2-3 xcexcm. The next step is coating of the particulate. When coated with liquid metal, these particles have liquid metal/boron nitride volume ratios ranging from 4:1 to 1:1. Coating is achieved by mechanically mixing as previously stated. This is followed by the addition of the appropriate amount of liquid or fluid silicone to the coated particulate, with this addition being followed by high-speed mixing until a visually smooth paste is obtained.
As indicated above, while boron nitride is the preferred particulate, favorable results have been achieved through the utilization of alumina, with the alumina typically requiring a pre-treatment which involves thorough drying of the particulate. Other particulates such as aluminum nitride can also form liquid metal pastes after thorough drying.
Therefore, it is a primary object of the present invention to provide an improved particulate material which in addition to being highly thermally conductive, functions to anchor a liquid metal into a three phase composite.
It is a further object of the present invention to provide an improved method of preparing a thermally conductive bridge between the opposed surfaces of a heat generating semiconductor device and a heat dissipating surface, with the thermally conductive bridge comprising a three phase composite consisting of inorganic particulate-liquid metal-liquid silicone polymer.
Other and further objects of the present invention will become apparent to those skilled in the art upon a study of the following specification, appended claims, and accompanying drawings.